1. Field of the Invention
The present invention relates to the fabrication of a single-crystal silicon by a continuously charged method, and more specifically, to an apparatus for detecting the diameter of the single-crystal silicon.
2. Description of Prior Art
Single-crystal silicon is often utilized as the substrate material of semiconductor devices. The single-crystal silicon can be fabricated by the Czochralski (CZ) method in which a cylindrical single-crystal silicon rod is pulled up from melt material inside a crucible which is located inside a fabricating chamber of a single-crystal silicon fabrication apparatus. Polysilicon material is charged into the crucible. The polysilicon material is heated and melted by a main heater disposed around the crucible. A single-crystal seed, which is fixed on a seed holder, is immersed in the melt. When the seed holder rises from the melt and rotates in phase or out phase with the rotation direction of the crucible, the single-crystal silicon rod is formed on the seed. A continuously charged method, by which a sufficient amount of polysilicon material is continuously fed into the crucible, can be applied in accompaniment with the CZ method to efficiently produce large-diameter single-crystal silicon rods.
The diameters of the single-crystal semiconductor material should be controlled with acceptable ranges when it is fabricated by the CZ method. The methods for controlling the diameters include optical methods, methods by weighting the crystal, and so on. In the optical method, a real-time measurement of the diameter can be carried out by instruments such as an image sensor and ITV camera. That is, a line that transverses the meniscus ring formed on an interface between the single-crystal material and the melt, and that passes through the center of the meniscus ring or passes through a point which has a specific distance to the center of the meniscus ring, is adapted as a scanning line of the image sensor. Pixel positions of the image sensor, which correspond to these lines, are detected to determine a distance between the pixel positions, and then the diameter of the single-crystal silicon is determined.
The single-crystal diameter measurement by line scanning or by one-dimensional scanning using the CCD camera cannot detect the precise diameter value in case that the melt surface is floating up and down since the scanning line is fixed. For example, referring to FIG. 5, when the melt surface position shifts down from position 3a to 3b, the detected diameter value of the single-crystal silicon 1 by a scanning line P will change from L1 to L2. Especially when the continuously charged method is utilized to provide the melt in the crucible, the position of the melt surface depends mostly on varying factors such as the charging amount of the material, the growing rate of the single-crystal silicon and the raising speed of the crucible. The material charged into the crucible will raise the height of the melt surface. The growing single-crystal silicon consumes the melt, thus lowering the height of the melt surface. The height of the melt surface obviously depends on the position of the crucible. Therefore, these varying factors must be controlled and detected for calibration of the scanning line position or the sensing angle, or the precise diameter of the growing single-crystal material cannot be obtained- Moreover, in the continuously charged method, after the single crystal pulling process continues for a certain period, the supply of the polysilicon material is stopped. However, since the melt surface falls gradually during this period, the conventional optical sensor cannot correctly detect the diameter of the single-crystal material. Therefore, the diameter of the single-crystal silicon must be estimated, based on the experiences of the operator.